Android 系统架构 —— 资源管理的创建

Resources res = getContext().getResources();
res.getString(R.string.xxx);

public final class LoadedApk {
    
    public Application makeApplication(boolean forceDefaultAppClass,
            Instrumentation instrumentation) {   
            
        // 构建 Application 对象
        if (mApplication != null) {
            return mApplication;
        }
        Application app = null;

        String appClass = mApplicationInfo.className;
        if (forceDefaultAppClass || (appClass == null)) {
            appClass = "android.app.Application";
        }

        try {
            java.lang.ClassLoader cl = getClassLoader();
            if (!mPackageName.equals("android")) {
                Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER,
                        "initializeJavaContextClassLoader");
                initializeJavaContextClassLoader();
                Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
            }
            // 1. 创建 Application 的 ContextImpl
            ContextImpl appContext = ContextImpl.createAppContext(mActivityThread, this);
            
            // 实例化 Application 对象
            app = mActivityThread.mInstrumentation.newApplication(
                    cl, appClass, appContext);
            appContext.setOuterContext(app);
        } catch (Exception e) {
            ......
        }
        .....// 回调 onCreate

        // 2. 重写 R 文件的 ID 值为运行时 ID
        SparseArray<String> packageIdentifiers = getAssets().getAssignedPackageIdentifiers();
        final int N = packageIdentifiers.size();
        for (int i = 0; i < N; i++) {
            final int id = packageIdentifiers.keyAt(i);
            if (id == 0x01 || id == 0x7f) {
                continue;
            }

            rewriteRValues(getClassLoader(), packageIdentifiers.valueAt(i), id);
        }
        return app;
    }
    
}

class ContextImpl extends Context {
    
    static ContextImpl createAppContext(ActivityThread mainThread, LoadedApk packageInfo) {
        ......
        // 1. 创建 ContextImpl 对象
        ContextImpl context = new ContextImpl(null, mainThread, packageInfo, null, null, null, 0,
                null);
        // 2. 通过 LoadedApk.getResources 来获取 Resource 对象
        // 3. 将 Resource 注入到 ContextImpl 中
        context.setResources(packageInfo.getResources());
        return context;
    }

}

public final class LoadedApk {
    
    Resources mResources;
    
    public Resources getResources() {
        if (mResources == null) {
            final String[] splitPaths;
            try {
                splitPaths = getSplitPaths(null);
            } catch (NameNotFoundException e) {
                ......
            }
            // 通过 ResourceManager 来获取这个 apk 对应的资源
            mResources = ResourcesManager.getInstance().getResources(null, mResDir,
                    splitPaths, mOverlayDirs, mApplicationInfo.sharedLibraryFiles,
                    Display.DEFAULT_DISPLAY, null, getCompatibilityInfo(),
                    getClassLoader());
        }
        return mResources;
    }
    
}

public class ResourcesManager {
  
    private static ResourcesManager sResourcesManager;
    
    public static ResourcesManager getInstance() {
        synchronized (ResourcesManager.class) {
            if (sResourcesManager == null) {
                sResourcesManager = new ResourcesManager();
            }
            return sResourcesManager;
        }
    }
   
    public @Nullable Resources getResources(@Nullable IBinder activityToken,
            @Nullable String resDir,
            @Nullable String[] splitResDirs,
            @Nullable String[] overlayDirs,
            @Nullable String[] libDirs,
            int displayId,
            @Nullable Configuration overrideConfig,
            @NonNull CompatibilityInfo compatInfo,
            @Nullable ClassLoader classLoader) {
        try {
            ...// systrace 埋点
            // 将请求的资源信息构建成 ResourceKey
            final ResourcesKey key = new ResourcesKey(
                    resDir,
                    splitResDirs,
                    overlayDirs,
                    libDirs,
                    displayId,
                    overrideConfig != null ? new Configuration(overrideConfig) : null, // Copy
                    compatInfo);
            // 获取 LoadedApk 传递过来的 ClassLoader
            classLoader = classLoader != null ? classLoader : ClassLoader.getSystemClassLoader();
            // 调用 getOrCreateResources 获取资源信息
            return getOrCreateResources(activityToken, key, classLoader);
        } finally {
            ...// systrace 埋点
        }
    }
    
}

public class ResourcesManager {

    private @Nullable Resources getOrCreateResources(@Nullable IBinder activityToken,
            @NonNull ResourcesKey key, @NonNull ClassLoader classLoader) {
        synchronized (this) {
            ......
            // 1. 从缓存中查找 ResourceImpl
            // 1.1 在构建 Activity 的 ContextImpl 的 Resource 是走这里
            if (activityToken != null) {
                final ActivityResources activityResources =
                        getOrCreateActivityResourcesStructLocked(activityToken);

                ......// 清除一些失效的引用
                
                // 根据 key 从缓存中查找 ResourceImpl 对象
                ResourcesImpl resourcesImpl = findResourcesImplForKeyLocked(key);
                if (resourcesImpl != null) {
                    ......
                    return getOrCreateResourcesForActivityLocked(activityToken, classLoader,
                            resourcesImpl, key.mCompatInfo);
                }
            } 
            // 1.2 在构建其他 ContextImpl 的 Resource 时走这里, 我们的 Application 会走这里
            else {
                
                ......// 清除一些失效的引用
                
                // 根据 key 从缓存中查找 ResourceImpl 对象
                ResourcesImpl resourcesImpl = findResourcesImplForKeyLocked(key);
                if (resourcesImpl != null) {
                    ......
                    // 获取 Resource 对象
                    return getOrCreateResourcesLocked(classLoader, resourcesImpl, key.mCompatInfo);
                }
            }

            // 2. 构建新的 ResourceImpl
            ResourcesImpl resourcesImpl = createResourcesImpl(key);
            if (resourcesImpl == null) {
                return null;
            }
            
            // 3. 添加到缓存
            mResourceImpls.put(key, new WeakReference<>(resourcesImpl));
            
            // 4. 创建 Resources
            final Resources resources;
            if (activityToken != null) {
                // 4.1 创建 Activity 的 Resources
                resources = getOrCreateResourcesForActivityLocked(activityToken, classLoader,
                        resourcesImpl, key.mCompatInfo);
            } else {
                // 4.2 创建其他的 Resource
                resources = getOrCreateResourcesLocked(classLoader, resourcesImpl, key.mCompatInfo);
            }
            return resources;
        }
    }
    
}

public class ResourcesManager {

    private final ArrayMap<ResourcesKey, WeakReference<ResourcesImpl>> mResourceImpls =
            new ArrayMap<>();

    private @Nullable ResourcesImpl findResourcesImplForKeyLocked(@NonNull ResourcesKey key) {
        // 从缓存中获取 ResourcesImpl
        WeakReference<ResourcesImpl> weakImplRef = mResourceImpls.get(key);
        ResourcesImpl impl = weakImplRef != null ? weakImplRef.get() : null;
        if (impl != null && impl.getAssets().isUpToDate()) {
            return impl;
        }
        return null;
    }    
    
}

public class ResourcesManager {

    private final ArrayList<WeakReference<Resources>> mResourceReferences = new ArrayList<>();

    private @NonNull Resources getOrCreateResourcesLocked(@NonNull ClassLoader classLoader,
            @NonNull ResourcesImpl impl, @NonNull CompatibilityInfo compatInfo) {
        // 1. 遍历 mResourceReferences 集合, 找寻与传入的 ResourcesImpl 和 ClassLoader 都相同的 Resources
        final int refCount = mResourceReferences.size();
        for (int i = 0; i < refCount; i++) {
            WeakReference<Resources> weakResourceRef = mResourceReferences.get(i);
            Resources resources = weakResourceRef.get();
            // ClassLoader 和 impl 都相同
            if (resources != null && Objects.equals(resources.getClassLoader(), classLoader) &&
                    resources.getImpl() == impl) {
                .......
                return resources;
            }
        }

        // 2. 创建新的 Resources 对象
        Resources resources = compatInfo.needsCompatResources() ? new CompatResources(classLoader)
                : new Resources(classLoader);
        // 2.1 注入 ResourcesImpl
        resources.setImpl(impl);
        // 添加到缓存集合
        mResourceReferences.add(new WeakReference<>(resources));
        ......
        return resources;
    }

}

public class ResourcesManager {
    
    protected @Nullable AssetManager createAssetManager(@NonNull final ResourcesKey key) {
        final AssetManager.Builder builder = new AssetManager.Builder();
        
        // 1. 获取 mResDir 的 ApkAssets 对象, 添加到 AssetManager.Builder 中
        if (key.mResDir != null) {
            try {
                
                builder.addApkAssets(loadApkAssets(key.mResDir, false /*sharedLib*/,
                        false /*overlay*/));
            } catch (IOException e) {
                ......
            }
        }

        // 2. 获取 mSplitResDirs 的 ApkAssets 对象, 添加到 AssetManager.Builder 中
        if (key.mSplitResDirs != null) {
            for (final String splitResDir : key.mSplitResDirs) {
                try {
                    builder.addApkAssets(loadApkAssets(splitResDir, false /*sharedLib*/,
                            false /*overlay*/));
                } catch (IOException e) {
                    ......
                }
            }
        }

        // 3. 获取 mOverlayDirs 的 ApkAssets 对象, 添加到 AssetManager.Builder 中
        if (key.mOverlayDirs != null) {
            for (final String idmapPath : key.mOverlayDirs) {
                try {
                    builder.addApkAssets(loadApkAssets(idmapPath, false /*sharedLib*/,
                            true /*overlay*/));
                } catch (IOException e) {
                    ......
                }
            }
        }
        
        // 4. 获取 mLibDirs 的 ApkAssets 对象, 添加到 AssetManager.Builder 中
        if (key.mLibDirs != null) {
            for (final String libDir : key.mLibDirs) {
                if (libDir.endsWith(".apk")) {
                    ......
                    try {
                        builder.addApkAssets(loadApkAssets(libDir, true /*sharedLib*/,
                                false /*overlay*/));
                    } catch (IOException e) {
                        ......
                    }
                }
            }
        }
        // 5. 构建 AssetManager
        return builder.build();
    }
    
}

public final class ApkAssets {
    
    @GuardedBy("this") private final long mNativePtr;
    @GuardedBy("this") private StringBlock mStringBlock;
    
    public static @NonNull ApkAssets loadFromPath(@NonNull String path, boolean system,
            boolean forceSharedLibrary) throws IOException {
        // 创建了 ApkAssets 实例
        return new ApkAssets(path, system, forceSharedLibrary, false /*overlay*/);
    }
    
    private ApkAssets(@NonNull String path, boolean system, boolean forceSharedLib, boolean overlay)
            throws IOException {
        // 1. 调用 nativeLoad 到 Native 层加载资源
        mNativePtr = nativeLoad(path, system, forceSharedLib, overlay);
        // 2. 调用 nativeGetStringBlock 获取资源包中的字符串常量
        // 3. 构件 StringBlock 对象
        mStringBlock = new StringBlock(nativeGetStringBlock(mNativePtr), true /*useSparse*/);
    }    
    
}

public final class AssetManager implements AutoCloseable {

    public static class Builder {
    
        private ArrayList<ApkAssets> mUserApkAssets = new ArrayList<>();
        
        public AssetManager build() {
            // 1. 合并系统 Apk 资源数据 和 刚刚解析的 Apk 资源数据
            
            // 1.1 获取系统 Apk 资源数据集合
            final ApkAssets[] systemApkAssets = getSystem().getApkAssets();
            
            // 1.2 合并数据到 apkAssets数组
            final int totalApkAssetCount = systemApkAssets.length + mUserApkAssets.size();
            final ApkAssets[] apkAssets = new ApkAssets[totalApkAssetCount];
            System.arraycopy(systemApkAssets, 0, apkAssets, 0, systemApkAssets.length);
            final int userApkAssetCount = mUserApkAssets.size();
            for (int i = 0; i < userApkAssetCount; i++) {
                apkAssets[i + systemApkAssets.length] = mUserApkAssets.get(i);
            }
            
            // 2. 构建 AssetManaager
            final AssetManager assetManager = new AssetManager(false /*sentinel*/);
            
            // 3. 将资源数组 apkAssets 保存到 Java 的 AssetManaager
            assetManager.mApkAssets = apkAssets;
            
            // 4. 将资源数组 apkAssets 保存到 Native 的 AssetManaager
            AssetManager.nativeSetApkAssets(assetManager.mObject, apkAssets,
                    false /*invalidateCaches*/);
            return assetManager;
        }
    }
    
    @GuardedBy("this") private long mObject;

    private AssetManager(boolean sentinel) {
        // 2.1 创建 Native 层的 AssetManager
        mObject = nativeCreate();
        ......
    }
    
    private static native long nativeCreate();
    
    private static native void nativeSetApkAssets(long ptr, @NonNull ApkAssets[] apkAssets,
            boolean invalidateCaches);
}

public final class AssetManager implements AutoCloseable {

    public static AssetManager getSystem() {
        synchronized (sSync) {
            // 获取系统的 AssetManager
            createSystemAssetsInZygoteLocked();
            return sSystem;
        }
    }
    
    private static final String FRAMEWORK_APK_PATH = "/system/framework/framework-res.apk";
    
    @GuardedBy("sSync") static AssetManager sSystem = null;

    
    /**
     * This must be called from Zygote so that system assets are shared by all applications.
     */
    @GuardedBy("sSync")
    private static void createSystemAssetsInZygoteLocked() {
    
        if (sSystem != null) {
            return;
        }
        
        ......

        try {
            final ArrayList<ApkAssets> apkAssets = new ArrayList<>();
            // 1. 加载系统资源 framework-res.apk 的数据
            apkAssets.add(ApkAssets.loadFromPath(FRAMEWORK_APK_PATH, true /*system*/));
            loadStaticRuntimeOverlays(apkAssets);
            sSystemApkAssetsSet = new ArraySet<>(apkAssets);
            sSystemApkAssets = apkAssets.toArray(new ApkAssets[apkAssets.size()]);
            
            // 2. 构建系统资源的 AssetManager
            sSystem = new AssetManager(true /*sentinel*/);
            
            // 3. 将数据保存到系统的 AssetManager
            sSystem.setApkAssets(sSystemApkAssets, false /*invalidateCaches*/);
        } catch (IOException e) {
            ......
        }
    }
    
}